(1) Field of the Invention
This invention relates to a solar cell system for storing electric power generated by solar cells, and supplying the power to a load.
(2) Description of the Related Art
A solar cell system used as a power source for an unmanned lighthouse, street light or the like includes a storage battery provided between solar cells and a load. Electric power generated by the solar cells is stored in the storage battery which supplies the load with power in a stable way even on rainy days and at night-time.
The solar cells and storage battery of this solar cell system have capacities determined based on solar radiation data of an installation site. This is because the quantity of solar radiation is variable with the latitude and seasons of the installation site.
The capacities are determined in the following sequence, for example.
Assuming that the load consumes electric power PL per day, output PS of the solar cells (in fine weather) is expressed by the following equation: EQU PS=K.times.PL.
In the above equation, correction factor K is variable with the installation site or region, and generally is selected from within the range of 1 to 4.
The relationship between the correction factor K and sunlight hours is derived empirically from results of actual operating tests and the like carried out on the most standard, independent power source combining solar cells and the storage battery. Correction factor K embraces variations with temperature changes of solar cell output occurring throughout the year, and correction values for the storage battery efficiency and the like.
Correction factor K is given the following values, for example:
______________________________________ sea, road centers, speedways, and poles 1-2 (high or sunny locations) buildings and other such structures 2-3 roadsides, grounds, and curbstones 3-4 ______________________________________
The capacity of solar cells for providing a current needed to charge the storage battery is determined from solar cell output PS derived as above.
The storage battery combined with the solar cells is required to meet the following conditions:
(1) small self-discharge;
(2) easy maintenance;
(3) long life;
(4) low price;
(5) large capacity per unit volume; and
(6) efficient charging and discharging.
Thus, the storage battery usually comprises a lead-acid battery which is inexpensive and provides an economic advantage, or a nickel-cadmium battery (so-called Ni-Cd battery) which has a long cycle life.
Capacity PB of the storage battery is determined from various conditions, such as a possible spell of sunless weather, and the number of days the system is desired to rely solely on the storage battery. Generally, capacity PB is determined to cover 5 to 30 days of power consumption PL by the load.
That is, battery capacity PB=5 to 30.times.PL.
With the conventional solar cell system, the solar cells and storage battery have large capacities to secure sufficient margins since the intensity of sunlight irradiating the solar cells is not constant but widely variable.
However, the conventional solar cell system is designed on the assumption that the solar cells must be installed in a sunny location (facing south), and not in the shade, to receive solar radiation in fine weather. This aspect has been an obstacle to widespread use of the solar cell system.
Further, as noted above, the solar cells and storage battery have large capacities determined to have sufficient margins. This results in a large construction and high cost, which again, obstructs widespread use of the solar cell system.
The storage battery (secondary battery) used in the solar cell system has a short charge/discharge cycle life of approximately 200-500 times, and must normally be changed every two or three years. Besides, a lead-acid battery requires periodic inspections such as replenishment and specific gravity measurement of the electrolyte, and hence its maintenance is troublesome. As a result, the system involves high maintenance and other costs.
Furthermore, the secondary battery includes heavy metals such as lead or cadmium. Disposal of the secondary battery leads to a secondary pollution, and therefore a necessity of its collection arises.
The secondary battery has a narrow use temperature range of 0.degree. to 45.degree. C. This is detrimental for using the system in a cold climate.